Sebastian Neiss

Analytical model for nonlinear piezoelectric energy harvesting devices

In this work we propose analytical expressions for the jump-up and jump-down point of a nonlinear piezoelectric energy harvester. In addition, analytical expressions for the maximum power output at optimal resistive load and the 3 dB-bandwidth are derived. So far, only numerical models have been used to describe the physics of a piezoelectric energy harvester. However, this approach is not suitable to quickly evaluate different geometrical designs or piezoelectric materials in the harvester design process. In addition, the analytical expressions could be used to predict the jump-frequencies of a harvester during operation. In combination with a tuning mechanism, this would allow the design of an efficient control algorithm to ensure that the harvester is always working on the oscillatorʼs high energy attractor.

Piezo-Polymer-Composite Unimorph Actuators for Active Cancellation of Flow Instabilities Across Airfoils

This article presents a smart device for active cancellation of flow instabilities. An array of two piezo unimorph actuators fabricated in piezo-polymer-composite technology is combined with a thin silicone membrane to mimic a movable wall with a closed surface. By locally displacing the thin membrane, a surface wave is generated that interferes with naturally occurring flow instabilities within the boundary layer of an airfoil. Using flow sensors and an intelligent control enables a destructive interference and therefore, an attenuation of natural flow instabilities. This leads to a delay of transition. The boundary layer remains laminar which means drag is reduced. Within the next pages, the setup of the device with actuators, membrane, sensors, and control is introduced. The main focus of this article is on actuator design, modeling, and implementation for wind tunnel experiments. Results of actuator characterization are presented. The non-linear behavior of the piezoactuator (harmonic distortions and impact of high electric fields) is investigated in detail. This study concludes with the results obtained in wind tunnel experiments which prove the functionality of the presented approach. A maximal attenuation of natural occurring flow instabilities of 80% is achieved.

Cymbal type Piezo-Polymer-Composite actuators for active cancellation of flow instabilities on airfoils

This paper presents the design and fabrication of a Cymbal type piezo actuator in Piezo-Polymer-Composite (PPC) technology. The purpose of the developed actuators is, in combination with a full setup including flow sensors, a digital control system, and an elastic membrane, an active manipulation of boundary layer disturbances at airfoils. The goal is to reduce the friction drag on an airfoil by delaying the transition from a laminar to a turbulent boundary layer. The presented results of wind tunnel experiments will prove the successful accomplishment of this task. Boundary layer instabilities, the cause for the transition, are dampened by more than 80 %.

Piezoelectric Materials for Nonlinear Energy Harvesting Generators

Nonlinear piezoelectric energy harvesting generators can provide a large bandwidth combined with a good resonant power output. In an experimental study, the influence of the piezoceramic material on these two parameters is investigated. The results prove hard piezoceramics to be better suited as converting element compared to soft piezoceramics. Their improved mechanical quality compensates for their low piezo-mechanical coupling leading to both, a larger bandwidth and a higher power output of the generator.

Self-sufficient electronic control for nonlinear, frequency tunable, piezoelectric vibration harvesters

Research in vibration energy harvesting focuses increasingly on nonlinear harvesters. In comparison to linear harvesters they show an inherent larger bandwidth through hardening or softening effects and higher conversion efficiency. A further increase of the bandwidth and thus a higher energy yield can be achieved by controlled tuning of such a nonlinear system. In this paper a self-sufficient tuning control electronic, which is directly powered by the harvester, is presented.

Tunable nonlinear piezoelectric vibration harvester

Nonlinear piezoelectric energy harvesting generators can provide a large bandwidth combined with a good resonant power output. However, the frequency response is characterized by a strong hysteresis making a technical use difficult if the hysteresis cannot be compensated. We propose a tuning mechanism that allows both, a compensation of the hysteresis as well as maintaining the optimal work point. The compensation algorithm can reduce the hysteresis to a minimum of only 1.5 Hz and maintain a high energy oscillation in a large frequency window between 53.3 Hz and 74.5 Hz.

Development and Fabrication of Active Microstructures for Wave Control on Airfoils

Transition of an airfoil’s boundary layer from the laminar to the turbulent flow regime increases the overall drag of an airplane significantly. The major origin of this transition are Tollmien-Schlichting (TS) waves. Similar to the dolphin’s skin, a system that is capable to dampen TS waves locally is proposed here. A surface wave can interfere destructively with TS waves and thus delay transition towards the edge of the airfoil. For this purpose, an actuator array is combined with a thin membrane. The presented actuators were developed and improved continuously so that all requirements for the dampening of TS waves are fulfilled. Several actuators are cascaded in a compact manner and combined with a membrane and an array of sensors. The system has proven in wind tunnel experiments to be capable of dampening TS waves successfully and delaying transition.